To make descriptions simple, descriptions of NED for one eye is used unless specified for pair of eyes.
Some NEDs are used in virtual reality (VR) devices. NEDs of VR devices have eyes enclosed from external view to effect immersive display. VR devices requires NEDs with a minimum of 80 degrees field-of-view (FOV) so that the viewer does not seem to be looking out from the window with the edges of the view showing. NEDs in VR devices use combination of lenses to focus on small display or image source. The problem is that this train of lenses are uncomfortably long, complex and bulky to achieve the minimum FOV and large eye box. An eye has a FOV of around 200 degrees horizontal and around 135 degrees vertical with eye rotation included (no head movement relative to NED). The pupil of the eye can be in locations covering around ¾ of an inch cube eye box with eye rotation and being slightly far or near the NED. NEDs having wide FOV and large eye box approaching these extents are ideal. Unfortunately, even the largest lens in NEDs that can be used in VR devices prohibits the NED from achieving a very wide FOV. Optical problems arise when the eye starts to rotate from the optical axis or move away from the focal plane. Attempts to solve these problems using concentric optical system did not produce satisfactory FOV or image quality. Prior concentric optical system for NEDs use combination of see-thru mirrors, polarizers, quarterwave retarders, and lens system to cut and fold the optical path. Unfortunately, light from the image source passes through and reflected from these elements so many times that the image quality becomes degraded so much.
There are NEDs used for see-thru or augmented reality (AR) devices. AR NEDs are similar to VR NEDs except AR NEDs can overlay the image to the external view. NEDs in AR devices have displays or image sources placed away from line of sight and use angled see-thru mirror or beam splitter to see the local display overlaid to the external view. These AR NEDs have unsatisfactory FOV. The angled see-thru mirror prohibits the NED from having wide FOV. Prior concentric optical system for NEDs of AR devices have failed to produce wide FOV with good image quality.
NEDs in AR devices have image from local display overlaid against the external view. Dark parts of the image unfortunately turn light when set against bright external view. The image could be very hard to see against brightly colored external view. Masking or occluding systems to cover parts or the whole image of the local display from external view to control the opacity against the external view have been tried. Unfortunately, these attempts have failed.
U.S. Pat. Nos. 5,517,366 and 5,659,430 both to Togino describes the advantages of using lenses in concentric optical system for NEDs to provide wide FOV and large eye box. Togino's embodiment uses beam-splitting mirrors, polarizers, and quarterwave retarders described in U.S. Pat. No. Re. 27356 to La Russa to fold and cut optical paths. La Russa's embodiment, however, greatly reduces the light finally reaching the viewer's eye thereby degrading the image so much.
U.S. Pat. No. 4,859,031 to Berman et al offered a solution to the beam-splitting mirror, polarizer, and quarterwave retarders combination problem of Togino's and La Russa's embodiment. By employing cholesteric liquid crystal element to the optical combinations, Berman was able to improve the image reaching the viewer's eye. With Berman's embodiment, however, the color of which the cholesteric liquid crystal is tuned is reduced by about 50% thereby degrading the transmitted image to the observer. The other 50% of the color is reflected back to the concave mirror then reflected back towards the cholesteric liquid crystal and then transmitted towards the observer. The transmitted color, however, is unfocused thereby degrading the image further. Another disadvantage is that the generated image which is overlaid to external view can only be in one color of which the cholesteric liquid crystal is tuned. Other colors from the image source which are not tuned will be transmitted by the cholesteric liquid crystal to the observer unfocused. Another disadvantage is that it has small FOV.
U.S. Pat. No. 7,639,208 to Ha et al. describes a compact see-through HMD with occlusion support. Ha's embodiment is a NED that is intended to be used for augmented reality applications. Ha's embodiment, however, has small FOV. The train of optical elements is very long. The external image reaching the eye is flipped right to left.
A NED for AR device using concentric optical system has been described by Rolf R. Hainich in his paper “Freeform Mirrors and Displays for AR” in sub-title “Approaches to Ideal Freeform Mirror and Display Shapes for Augmented Reality”. He describes a NED using spherical display in combination with concentric optical system. His description is that the display should be emitting outward away from the eye or on the convex side of the display so that the image is reflected back by the concentric concave spherical mirror. Part of the reflected image then passes through the spaces between the pixels then towards the eye which is behind the display. The spaces, however, removes the pixels from the display to allow the image to pass through thereby reducing the display resolution. The pixels at the display also blocks part of the reflected image thereby reducing the image further. Hainich's description of the NED does not describe an occluding system.